Method and medium for detecting Shiga toxin-producing Escherichia coli bacteria

ABSTRACT

The present invention relates to a method for specific and direct detection of Shiga toxin-producing  Escherichia coli  bacteria in a sample using a selective and differential isolation medium for Shiga toxin-producing  Escherichia coli  bacteria comprising at least one chromogenic agent and tellurite.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Stage of International ApplicationPCT/EP2010/067226, filed Nov. 10, 2010, which claims the benefit ofFrench Patent Application No. 09 05415, filed Nov. 10, 2009, both ofwhich are hereby incorporated herein in their entirety by reference.

This invention relates to a process for specific and direct detection ofShiga toxin-producing Escherichia coli (STEC) bacteria in a sampleimplementing a selective and differential isolation medium for STECstrains comprising at least one chromogenic agent and tellurite.

The Shiga toxin-producing E. coli (STEC) are responsible for foodbornetaxi-infection that result in diarrhea as well as more serious syndromesin humans such as hemolytic uremic syndrome, which can provoke death.These are zoonotic agents of which the main reservoir is cattle andother ruminants. The main modes of transmission of STEC infections tohumans are consumption of contaminated foods (rare beef, unpasteurizeddairy products), transmission from person-to-person, ingestion ofcontaminated water and contact with animals (in particular cattles) andtheir environment.

More specifically, the STEC strain of serotype O157:H7 is responsiblefor epidemics throughout the world. Numerous diagnostic methods havebeen developed in order to identify this pathogen from foods. Theseinclude conventional bacteriology methods, immunological methods andmolecular methods. Hygiene measures are particularly important forpreventing contamination of farm animals and meat in the slaughterhouse.Finally, risk assessment models have been developed in particular inorder to model the behavior of STEC in food.

Although E. coli O157 is the most common STEC, other strains arefrequently involved in human pathologies. Among these strains, mentionmay be made of O26, O103, O111 and O145. Few laboratories conduct testsfor detecting these non-O157 STECs due to the lack of standardizedmethods and the cost of the complex equipment necessary for conductingsuch tests, such as modern PCR methods.

Among the isolation media of the prior art, mention may be made of thesorbitol MacConkey (SMAC) medium, which was developed in order toisolate the negative O157 strains in sorbitol. However, non-O157 strainsor positive O157 strains in sorbitol cannot be isolated by using thismedium. Modifications of the SMAC medium were proposed and essentiallymake it possible to increase the selectivity of negative E. coli O157 byusing inhibitors and/or antibiotics such as CT-SMAC, in which potassiumtellurite and cefixime are added to the SMAC medium, and the CR-SMACmedium, in which the cefixime and the rhamnose are added to the SMACmedium in order to improve the efficacy of isolation of O157 strains.

The methods of the prior art using a medium for the isolation ofnon-O157 STEC strains comprise one or more sugars or alcohols. Suchmedia do not however make it possible to obtain sufficient sensitivityand also lead to false positives, limiting the reliability of theresults.

It is therefore important to have tools and processes for detectingthese bacteria, which combine both good specificity and selectivity, andin particular ease of use so that it is possible to simplify the testsas much as possible, to conduct them quickly and in large numbers, evenautomate them, with the objective of ensuring food and/or hospitalhygiene, while enabling different STEC strains to be differentiatedquickly.

There is therefore a real need to have a simpler, more specific, moredirect and less expensive detection technique making it possible todifferentiate different STEC strains while avoiding combining aplurality of tests, producing an additional delay in obtaining resultsand increasing the risk of parasitic contaminations or error as well asthe risk of spread of the bacteria.

Surprisingly and unexpectedly, the Inventor has shown that the use oftellurite and a chromogen makes it possible to quickly isolate the STECsin a sensitive and specific manner. In particular, when it isimplemented on a solid agar medium, the detection process developed bythe Inventor can be carried out directly, for example from a food sampleor from a sample collected from a patient, without requiring apreliminary step of isolation of the different strains present in saidsample.

In particular, the process according to the invention can be applied tothe detection of STEC strains chosen from the group consisting ofstrains O26, O103, O111, O145 and O157 and even more preferably from thegroup consisting of strains O26, O103, O111 and O145.

This invention therefore relates to a process for direct detection ofShiga toxin-producing Escherichia coli bacteria in a sample comprisingthe successive steps of;

a) inoculation, with said sample, of a culture medium includingtellurite and at least one chromogenic agent,

b) incubation of said culture medium under conditions enabling thegrowth of Shiga toxin-producing Escherichia coli bacteria, and

c) detection of Shiga toxin-producing Escherichia coli bacteria coloniesformed on said culture medium.

Advantageously, the process according to the present invention furthercomprises a step d) making it possible to conclude whether or not aparticular strain of bacteria is present according to the color of thecolonies formed. This is therefore a step of d) identification of theSTEC strain in said sample.

With respect to the previous processes, the process developed by theInventor enables direct and quick detection of Shiga toxin-producingEscherichia coli (STEC) bacteria.

By “biological sample”, it is understood any type of microbiologicalsample, such as, for example, a sample of food material (dairy products,meat, etc.), a soil sample, a sample from a mammal (skin, mucous, etc.),preferably human, or one of its derivatives such as a pre-cultureobtained from such a sample.

Advantageously, said biological sample is a liquid biological sample,such as saliva, blood or urine, a solid biological sample, such as fecesor a food product, or a derivative of a liquid or solid biologicalsample such as a pre-culture of such a liquid or solid biologicalsample.

Also advantageously, said biological sample includes differentmicroorganisms, which may belong to species and even to distinct genera.By way of example, said biological sample includes at least twodifferent microorganisms, preferably at least five differentmicroorganisms, and, particularly preferably, at least ten differentmicroorganisms.

By “culture medium”, we mean a medium enabling the growth of said atleast one specific microorganism to be detected. Said culture mediumindeed includes the nutrients necessary for the growth of said at leastone specific microorganism to be detected.

By “nutrients necessary for the growth of said at least one specificmicroorganism to be detected”, we mean the composition of a basic mediumnecessary for said growth. A person skilled in the art fully knows thecomposition of such media and is capable of adapting it if necessaryaccording to the specificity of certain microorganisms or constraintsthat may be associated with certain cases of this invention(transparency of the medium, for example). These nutrients are inparticular chosen from the group including carbon, nitrogen, sulphur,phosphorus, vitamins, growth inducers, carbohydrates, salts (forexample, calcium, magnesium, manganese, sodium, potassium), nutritionalcomplexes (for example, amino acids, blood, serum, albumin) as well aspeptones and animal and plant tissue extracts.

The culture medium used in the context of the present invention for thedetection of STEC strains, and which constitutes another subject-matterof this invention, may be in solid, semi-solid, liquid or lyophilizedform. Preferably, said culture medium is an agar medium, and is, by wayof example, agar-based. Among the presentations of culture media thatcan be used, mention can thus be made of Petri dishes in whichmicroorganisms are developed.

The culture media according to this invention can optionally contain oneor more antimicrobial agents, in particular one or more antibioticsand/or one or more antifungal agents.

Said antimicrobial agent(s) make it possible to limit the growth ofmicroorganisms other than said at least one specific microorganism to bedetected.

The effective amount of antimicrobial agent to be used can be determinedsimply by a person skilled in the art owing to his or her generalknowledge.

By “culturing”, it is meant inoculating said culture medium with all orsome of the biological sample and incubating said inoculated culturemedium.

A person skilled in the art will adapt the incubation conditionsaccording to the culture medium, the biological sample and the specificmicroorganism to be detected according to his general knowledge.

The incubation step can be performed at a temperature of around 30° C.to 43° C., preferably 37° C., for a period of around 18 to 24 hours.However, depending on the means available, a person skilled in the artmay adapt the temperature and the duration of this incubation step inview of his general knowledge.

By “direct detection process”, it is meant a process that does notinclude a preliminary step of isolating the different bacterial strainspresent in the sample, preferably a process that does not comprise apreliminary step of isolating each of the bacterial strains present inthe sample.

Indeed, the process according to the invention makes it possible toavoid the step of isolating candidate bacterial colonies which can ofthen be subjected to a more precise test of confirmation of the STECproperty. It therefore applies to a raw sample comprising a mixture ofbacteria.

Preferably, the tellurite comprised in said medium is in the form ofpotassium tellurite.

By “chromogenic agent”, it is meant a compound having a precipitatingchromophore released after hydrolysis by a specific enzyme. Thechromophore thus released gives its color to the colonies including saidenzyme.

For example, among the enzymes of which the activity can be used in thecontext of the present invention, mention may be made in particular of:alpha-galactosidase, beta-D-glucuronidase, beta-D-galactosidase,C8-esterase, beta-glucosidase and beta-glucosaminidase, preferably saidenzyme is beta-glucosidase.

Advantageously, the medium according to this invention will comprise atleast one chromogenic agent sensitive to the activity ofbeta-glucosidase and at least one other enzyme chosen fromalpha-galactosidase, beta-D-glucuronidase, beta-D-galactosidase,C8-esterase, and beta-glucosaminidase, preferably alpha-galactosidase.

Preferably, the medium according to the present invention will includeat least one chromogenic agent sensitive to the activity ofbeta-glucosidase, and at least one chromogenic agent sensitive to theactivity of alpha-galactosidase.

Preferably, the medium according to the present invention will includedeoxycholate, and more preferably sodium deoxycholate.

Advantageously, the tellurite concentration in the medium according tothe present invention is between 0.5 and 10 mg/L, preferably between 1and 5 mg/L and even more preferably about 2.5 mg/L.

Advantageously, the concentration of chromogen(s) in the mediumaccording to the present invention is between 10 and 300 mg/L,preferably between 20 and 200 mg/L and even more preferably about 100mg/L.

The effective amount of tellurite and chromogen(s) in the mediumaccording to the present invention may simply be adjusted by a personskilled in the art in view of his or her general knowledge and theresults described in the examples below.

The incubation conditions enabling the growth of STEC strains are wellknown to a person skilled in the art and are not different from those oftraditional methods.

It is possible, for example, to choose, as the culture medium used inthe process according to this invention, a CHROMagar™ Orientation medium(CHROMagar, Paris, France), to which tellurite and deoxycholate havebeen added.

Another objective of the invention relates to the use of a culturemedium as described above for the direct detection and differentiationof STEC strains.

The following example is provided by way of illustration and cannotlimit the scope of this invention.

EXAMPLE Direct Detection of Shiga Toxin-Producing Escherichia ColiBacteria by Means of a Medium According to the Present Invention

Different samples containing bacteria suspensions are spread directly ina Petri dish including an agar medium including:

-   -   peptone and yeast extract 8 g/L;    -   NaCl 5 gL;    -   agar 15 g/L;    -   X-glucoside 0.01 g/L;    -   Mag-alpha-galactoside 0.01 g/L;    -   sodium deoxycholate 1 g/L; and    -   tellurite 0.0025 g/L.

After incubation for 18 to 24 hours, the visual analysis of the Petridishes enables those including STEC strains to be identified directly.

The results are indicated in table 1 below:

TABLE 1 Size of colonies Strain isolated Color in mm E. coli O157 H7Mauve 1.5 E. coli O26 Mauve 1.5 E. coil O103 Mauve Irregular E. coliO111 Mauve 1 Klebsiella Blue 2 pneumoniae ND20 Proteus AR3919 Green 0.5Enterobacter — — cloacae ND36* E. coli AR3740* — — Proteus AR5075 Brown0.8 *Strains with inhibited growth

Thus, the process according to the invention makes it possible, in asingle culture step, to specifically detect and differentiate Shigatoxin-producing Escherichia coli bacteria, without the need for apreliminary isolation step or a subsequent differentiation step.

The invention claimed is:
 1. A method for direct detection of Shigatoxin-producing Escherichia coli bacteria chosen from the groupconsisting of O26, O103, O111 and O145 strains in a sample comprisingthe successive steps of: a) inoculation, with said sample, of a culturemedium comprising tellurite and at least one chromogenic agent sensitiveto the activity of beta-glucosidase, wherein said culture medium doesnot contain sorbitol, b) incubation of said culture medium underconditions enabling the growth of Shiga toxin-producing Escherichia colibacteria chosen from the group consisting of O26, O103, O111 and O145strains, and c) detection of colonies formed on said culture mediumcorresponding to Shiga toxin-producing Escherichia coli bacteria chosenfrom the group consisting of O26, O103, O111 and O145 strains.
 2. Themethod of claim 1, wherein said method does not comprise a preliminarystep of isolating the different bacterial strains present in the sample.